Device for treating wetting liquid in printing presses, particularly for offset printing presses

ABSTRACT

A method and device for treating the wetting liquid used to wash ink bearing portions of printing presses, particularly offset printing presses. The wetting liquid is based on water and has preset levels of acidity in electrical conductivity. The wetting liquid is subjected to electrolysis in a tank which may be separate from the water pan. Alternatively both the water pan and the ink bearing portion of the printing press may be electrically conductive, and connected to a DC power supply, so that both the washing and the electrolysis can be carried out in the electrically conductive water pan. The method increases the speed at which the ink dries in the printing phase and thereby enhances the efficiency of the printing press.

BACKGROUND OF THE INVENTION

The object of the present invention is a process and a device fortreating the wetting liquid in printing presses, particularly for offsetprinting presses.

As we know, the efficiency of printing presses, especially offsetprinting presses, is related mainly to the speed at which the ink usedfor the printing dries.

The task of increasing the ink drying speed is thus a particularly acuteproblem in this area.

To carry out printing by the so-called "offset" method, the cylinderthat holds the plate is inked by the inking cylinder and does theprinting on a rubber cylinder which, in turn, transfers the printing tothe sheet held by the printing cylinder.

The inking cylinder, in addition to being coated with ink, is "bathed"with a liquid that is usually composed simply of water, or a mixture ofwater and alcohol.

In the kind of printing that uses water as a wetting liquid, there-isthe problem of low drying speed, which makes it impossible to reach highhourly production rates. In addition, the use of water causes smearingof the plate, resulting in poor-quality printing.

The kind of printing that uses water plus alcohol as a wetting liquidcan achieve considerably better production rates in terms of bothquality and quantity since alcohol, by reducing the surface tension ofthe water, makes it possible to distribute a finer layer of liquid andink over the plate and thus ensures fewer smearing problems and a fasterdrying speed, facilitated by the volatility of the alcohol.

Along with these advantages, the use of water and alcohol as a wettingliquid poses some problems. As a matter of fact, the alcohol dilutes theink, reducing the contrast effects in printing.

Moreover, an ink blurring effect that considerably degrades the qualityof the printing is very commonly encountered.

Another problem is posed by the need for the machines with water-alcoholwashing to have devices for cooling the alcohol in order to keep it fromevaporating excessively.

An additional problem is the reduced tolerance of the personneloperating these machines for the alcohol vapors.

SUMMARY OF THE INVENTION

The primary task of the present invention is to solve the problemsoutlined above by devising a process and a device for treatingprinting-press wetting liquid that make it possible to eliminate the useof alcohol in the wetting liquid without impairing, and in fact directlyincreasing, the drying speed and thus the efficiency of these machines.

Within the framework of this task, one goal of the invention is todevelop a process that makes it possible to achieve a printing qualitythat is better than that which can be obtained using water and alcoholas a wetting liquid.

Another goal of the invention is to propose a process that avoids thesmearing problems which arise from using water alone as a wettingliquid.

A further goal of the invention is to propose a process that is simpleto implement and that does not create problems of work environmentcontamination.

This task, as well as those and others that are presented in greaterdetail below, are accomplished by means of a process for treating thewetting liquid in printing presses, particularly offset printingpresses, wherein said process consists of subjecting the wetting liquid,which is based on water and has preset levels of acidity and electricalconductivity, to electrolysis.

The process of the invention can be easily implemented using a device,wherein said device consists of a treatment tank, means of supplyingthis treatment tank with a wetting liquid that is based on water and haspreset levels of acidity and electrical conductivity, and adirect-current d.c.! electric power supply that is connected to an anodeand a cathode that are immersed in the wetting liquid contained in thistreatment tank for electrolysis of the wetting liquid placed in thistreatment tank.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention are presented ingreater detail in the description in the form of a preferred, but notexclusive, embodiment of the process, as well as of the device of theinvention, which are illustrated as examples and without limiting theirscope, in the attached drawings, where:

FIG. 1 it is a schematic of a device for carrying out the electrolysisof the process of the invention;

FIG. 2 shows a scheme for applying the device to implement the processof the invention to a printing press;

FIG. 3 shows another scheme for applying the device in which theelectric power supply is composed of a d.c. generator with its polesattached to the treatment tank and to a pan roller.

DESCRIPTION OF PREFERRED EMBODIMENTS

The process of the invention basically consists of using, as the wettingliquid, water that is desalinated and demineralized in advance by meansof, e.g., a reverse-osmosis filtration process, and to this water areadded acidity adjusting agents and electrical-conductivity adjustingagents in such a way as to bring the acidity and electrical conductivityof the wetting liquid to preset values. These preset values basicallyconsist of a pH of between 4.5 and 5.5 and an electrical conductivity ofbetween 800 μS and 2200 μS.

The acidity and electrical conductivity can be adjusted independently ofone another by using, for example, an acidity adjusting agent such as H₂SO₄, or HF, or citric acid, and an electrical conductivity adjustingagent such as NaCl, or by using concentrated acid salts simultaneously.

The wetting liquid with this level of acidity and electricalconductivity value can then be subjected to electrolysis using agraphite anode, or a carbon anode, or a platinum-plated titanium meshanode, and at least one steel cathode, preferably a stainless-steel one.

The power-supply voltage of the anode and the cathode are varied ininverse proportion to the conductivity of the wetting liquid and canvary essentially between 58 V and 38 V. In practice, with awettingliquid conductivity equal to 800 μS, a voltage of 58 V is used,and this voltage is progressively reduced as the conductivity of thewetting liquid increases until it is brought to a value of 38 V, atwhich point the wetting liquid reaches an electrical conductivity equalto 2200 μS.

The wetting liquid is preferably recirculated in a closed circuit froman electrolytic treatment tank to the water pan mounted on the press,and vice versa.

To implement the above-described process, a device is employed which canbe readily composed, as shown in particular in FIG. 1, of a treatmenttank 1, to which is sent the wetting liquid based on water with presetlevels of acidity and electrical conductivity, as described above. Intreatment tank 1 are immersed an anode 2, preferably made of graphite,and at least one cathode 3, which are connected to a d.c. power supply;this power supply can simply consist of a rectifier, of a known type,that is connected to the power grid.

Treatment tank 1 can be made of an electrically insulating material orsteel and can itself act as the cathode in the electrolysis process thatis carried out inside the treatment tank. In addition, this treatmenttank can also be the printing press's own water pan.

As FIG. 2 shows, the device for carrying out the process of theinvention can consist of a treatment tank 1a that is distinct from waterpan 5 which is mounted on the printing press and which is connected tothe latter via a closed recirculation circuit with a pump 6 for pumpingthe wetting liquid through the circuit.

In this case as well, the device consists of a d.c. power supply 4a thatis connected to an anode 2a, preferably made of graphite, and at leastone cathode 3a, preferably made of stainless steel, which are immersedin the wetting liquid of treatment tank 1a. To keep any impurities fromcirculating, a weir 8 is placed inside the treatment tank.

As we know, in offset printing presses the essential parts include atleast one pan roller, described above, which is colloquially referred toas a "bagnino" pan roller! and which serves to wet the printing platebefore the ink is applied to it. Since this wetting can be done withwater or, better yet, with the aqueous solution for the treatment forwhich the process of the invention has been developed, the inventor hasalso devised a considerably simplified device that can be readilyapplied to existing machines. In such a simplified device (FIG. 3), thepower supply consists of a d.c. generator 4b which is equipped with apotentiometer for regulation and whose shaft rotates as a single piecewith one of the different rotating parts of the machine in which thedevice is used.

In this embodiment above-described treatment tank 1b serves, moresimply, as the water pan for the printing press, into whichabove-mentioned wetter or "attendant" 9 extends, as usual; the lattershould be made of a conductive material, e.g., preferably chrome-facedsteel. The positive pole of the generator is connected to theabove-mentioned water pan which, since it is also made of a conductivematerial, should be insulated with respect to the outside and thus actsas the anode, while the negative pole is connected to "attendant" roller9, which serves as the cathode. It is thus possible to implement theprocess of the invention, with the same advantages as described-above,using a simpler and cheaper device.

The electrolysis that is applied to the wetting liquid with controlledlevels of acidity and electrical conductivity causes an increase in theconcentration of oxygen in the wetting liquid, which is identified asthe factor that causes the ink, in the printing phase, to dry inconsiderably shorter times than are required for drying in traditionalkinds of printing that use just water or water mixed with alcohol aswetting liquids. This phenomenon is based on the fact that the inkswhich are currently being used for printing, particularly for offsetprinting, have an elevated capacity for absorption and oxidation.Because of the electrolysis to which it is subjected, the heavilyoxygen-enriched wetting liquid makes it possible for the ink to absorblarge quantities of oxygen and thus causes it to oxidize faster and thusto dry more quickly.

In the case where sodium chloride is used as the agent for adjusting theelectrical conductivity of the wetting liquid, another factor thataccounts for the faster drying that is achieved with the wetting liquidthat is treated with the process of the invention is the fact that,during the electrolysis process, sodium peroxides are formed whichsubsequently enhance the oxidizing effect of the wetting liquid on theink.

In addition, since the plate that is used for offset printing isgenerally anodized, the wetting liquid, which is rich in ions due to theelectrolysis process, adheres better to the plate, which makes itpossible to have a constant and much thinner layer of liquid and ink onthe plate, thus avoiding smearing problems.

It has been confirmed in practice that the process of the inventioncompletely accomplishes the task at hand since it makes it possible toensure considerably faster drying of the ink used for printing than canbe achieved with the traditional use of water or water mixed withalcohol.

Another advantage lies in the fact that, with the process of theinvention, the use of alcohol in the wetting liquid is eliminated,resulting in considerably better printing quality.

The elimination of alcohol from the wetting liquid also makes itpossible to simplify the printing press since the devices for coolingthe alcohol are no longer needed and hazardous substances are no longerreleased into the work environment.

As designed, the process, as well as the device for its implementation,can have many modifications- and variations, all within the scope of theinventive concept; in addition, all of the parts can be replaced withother parts that are technically equivalent.

I claim:
 1. A device for treating a wetting liquid for an offset printing press, comprising:a treatment tank (1, 1a, 1b) in an offset printing press, means for supplying said treatment tank with a water-based printing press wetting liquid, which has preset levels of acidity and electrical conductivity, and a d.c. power supply (4a) that is connected to an anode (2a) and a cathode (3a) in contact with the wetting liquid contained in said treatment tank for electrolysis thereof.
 2. The device according to claim 1, wherein said anode (2a) comprises graphite.
 3. The device according to claim 1, wherein said anode (2a) comprises carbon.
 4. The device according to claim 1, wherein said anode (2a) comprises platinum-plated titanium mesh.
 5. The device according to claim 1, wherein said cathode (3a) comprises steel.
 6. The device according to claim 1, wherein said treatment tank (1a) comprises an electrically insulating material.
 7. The device according to claim 1, wherein said treatment tank (1a) comprises steel and constitutes said cathode.
 8. The device according to claim 1, wherein said d.c. power supply (4, 4a) comprises a rectifier powered from a power grid.
 9. The device according to claim 1, wherein said treatment tank (1b) comprises a water pan of the printing press and wherein the power supply is a d.c. generator (4b), whose positive pole is connected to said water pan (1b) made of a conductive material and whose negative pole is connected to an attendant roller (9), which is made of a conductive material and extends into the wetting liquid contained in said water pan (1b).
 10. The device according to claim 9, wherein said attendant roller (9) comprises chrome-plated steel.
 11. A device for treating a wetting liquid for a printing press, comprising:an electrically conductive water pan for holding a water-based printing press wetting liquid which has preset levels of acidity and electrical conductivity; an electrically conductive attendant roller in said water pan for contacting the wetting liquid in said water pan; and a d.c. power supply having a first terminal connected to said water pan and a second terminal connected to said attendant roller, for electrolysis of the wetting liquid in the water pan.
 12. A device for treating a wetting liquid for a printing press, comprising:a treatment tank for the printing press having a water pan made of a conductive material; means for supplying said water pan with a water-based printing press wetting liquid, which has preset levels of acidity and electrical conductivity; and a d.c. generator that is connected to an anode and a cathode in contact with the wetting liquid contained in said water pan for electrolysis thereof, wherein said generator's positive pole is connected to said wetting tank and negative pole is connected to an attendant roller that is made of a conductive material and extends into the wetting liquid contained in said water pan. 